The study is intended to test the hypothesis that secondary urbanization embodies changes in social and economic organization which will facilitate rapid fertility decline. This hypothesis amplifies and extends the recent argument by Freedman and Berelson that a "modernizing social setting" in association with intensified family planning operates to promote rapid fertility decline. The proposal argues that the growth of regional cities in S.E. Asia embodies the major factors operating to improve social settings and establish essential preconditions for acceptance of effective family planning. Davao City (3rd largest in the Philippines) has been chosen for a prospective longitudinal study of the relationship between changing socio-economic organization and fertility under circumstances of extremely rapid urban growth (5.5 percent per year). A baseline survey was completed in 1972 and replicated in 1974 on a 1,000-household sample. A third wave (with a sample of 3,500) is presently (1976) being completed. This proposal requests support for two years to (1) prepare a refined demographic, social and economic analysis of the 1976 Davao survey; (2) construct a time-series description of trends in relevant variables over the three surveys (1972-1974-1976); (3) plan and conduct a final fourth-round survey (3,800 households) in 1978 which will coincide with the National Demographic Survey of the Philippines.